Preparation And Properties Of Spinel Lithium Manganate Cathode Materials By Coprecipitation-Microwave Sintering Method

With the aggravation of global warming and pollution of fossil fuels,the development of lithium-ion battery technology has become increasingly important.Spinel lithium manganate cathode material is considered as the most potential material to replace lithium cobalt oxide due to its advantages of low cost,high operating voltage,safety,no pollution and high specific capacity.However,the capacity attenuation of spinel lithium manganate in the cycle progress has not been effectively solved,which seriously impedes its commercialization.Microwave heating technology has become more and more widely used due to its unique heating principle and characteristics.Structure design and ion doping of spinel lithium manganate by microwave heating technology can effectively improve its electrochemical capacity and cycling stability.In this thesis,the porous spinel lithium manganate material was prepared by coprecipitation-microwave sintering method using lithium nitrate,manganese nitrate and ammonium carbonate as raw materials.The effects of sintering temperature,sintering method,sintering time and lithium-manganese ratio on the prepared lithium manganate materials were studied.And the optimum lithium-to-manganese ratio was 1.05:2,the optimum sintering temperature was 850°C,and the optimum sintering time was 45 minutes.The finally obtained material is a porous spinel lithium manganate powder with a single crystal form and a particle size of 2 to 5?m.Combined with the characteristics of sitering method and preparation process,the reasons for the formation of porous morphology of lithium manganate materials were analyzed and discussed.On the one hand,it was caused by the heterogeneous microwave absorption properties of the precursor powder and the selective heating characteristics of microwave sintering.On the other hand,the precursor released a large amount of gas during the sintering process to form the porous structure.The thermogravimetric effect,crystal form,morphology and pore structure of lithium manganate were characterized by several experimental methods,such as TG-DSC,XRD,SEM and BET.The results show that the unit cell parameters of the porous spinel lithium manganate material are a=8.2438?,Vol=560.25?³.The crystallinity is 97.65%,the porosity is 77.8%,and the specific surface area is 22.77 m²/g.These data are superior to the lithium manganate materials prepared by conventional sintering methods.The prepared lithium manganate materials were tested for electrochemical performance such as first charge and discharge,cycle performance,rate performance,CV,and EIS.The results show that the first charge specific capacity of porous lithium manganate material is142.5 mAh/g,the first discharge specific capacity is 132.1 mAh/g.And after circulating 100times at 0.5C,there is still 74.7%capacity retention.There is still a discharge specific capacity of 92.4 mAh/g at a high rate of 10C,and its D_Li is as high as 2.128×10^-7cm²/s.These data are substantially superior to lithium manganate materials prepared by conventional sintering methods.Then,the method of structure design of the spinel lithium manganate materials by microwave sintering is extended to the preparation of ion doped modified spinel lithium manganate materials.Finally,porous spinel lithium nickel manganese oxide material was prepared and its structure and properties were characterized.The results show that the crystal structure of porous lithium nickel manganese oxide is more stable.It has a first charge specific capacity of 142.1 mAh/g and a first discharge specific capacity of 136.5 mAh/g.The capacity retention rate is 94.7%after 100 cycles,and the capacity retention rates are all above85%under high rate cycles.The electrochemical performance is significantly improved relative to lithium manganate materials.